(591) Support for Universal Third Trimester Ultrasound from Risk Factor-Based Analysis

Abnormalities of amniotic fluid or growth are linked with adverse outcomes; however, in low-risk pregnancies the majority are unidentified prior to delivery. This study aimed to identify maternal risk factors associated with oligohydramnios, polyhydramnios, fetal growth restriction (FGR), and large for gestational age (LGA) in low-risk pregnancies without any other indications for third trimester ultrasound.

Study Design:

This secondary analysis includes 661 low-risk pregnancies in the intervention arm from a previous study (PMID: 38583714) where all patients had fetal growth and amniotic fluid assessed by ultrasound at 36 weeks. Maternal factors (race, age, height, pre-pregnancy BMI, nulliparity, and insurance status) were analyzed in relation to the incidence of oligohydramnios, polyhydramnios, FGR, and LGA.

Results:

Of the patients analyzed, 147/661 (22.2%) had a composite of the four ultrasound findings. No single risk factor was predictive of this abnormal ultrasound composite (Table). When the composite components were analyzed separately, maternal age ≤19 years old (OR 5.49 95% CI: 1.72-17.56, p = 0.006) and nulliparity were associated predictors of oligohydramnios (OR 2.35 95% CI: 1.09-5.05, p = 0.025). Finally, patients who self-identified as black had lower rates of LGA at 36 weeks compared to white patients (OR 0.23 95% CI: 0.09-0.58, p < 0.001).

Conclusion:

At 36 weeks ultrasound exam composite of fetal growth or amniotic fluid abnormality was identified in 1 in 5 low-risk pregnancies, with no significant predictive factors apparent prior to universal screening. This supports implementing a universal 36-week ultrasound screening protocol for low-risk pregnancies. The association of oligohydramnios and LGA with maternal age, parity, and race warrants further study. When routinely offered to all patients, third trimester ultrasound facilitates the identification of abnormalities that would otherwise go undetected with current recommended fundal-height protocols.